What was left was a muddled draw with one principal attraction: a possible semifinal matchup between top-seeded Rafael Nadal and No. 3 Roger Federer, who had never played at the U.S. Open. When Federer lost in the quarterfinals, that hope was dashed, too.

But with all the chaos and general lack of star power, some semblance of order Friday was maintained when Nadal reached the U.S. Open final again.

In a semifinal that many felt was the de facto final, Nadal mercilessly ran No. 24 Juan Martín del Potro of Argentina into exhaustion, 4-6, 6-0, 6-3, 6-2, in Arthur Ashe Stadium to reach his 23rd Grand Slam final. On Sunday, Nadal will face No. 28-seeded Kevin Anderson of South Africa, who will be playing his first Grand Slam final.

Anderson, 31, beat the 12th-seeded Pablo Carreño Busta, 4-6, 7-5, 6-3, 6-4, in the first semifinal of the day, which had a distinct warm-up act feel to it.

The marquee event came later in an intriguing matchup between del Potro, the 2009 U.S. Open champion still working his way back from multiple wrist surgeries, and Nadal, a winner here in 2010 and 2013 who is starting to look as good as he did in those years.

Image

Kevin Anderson in his semifinal win over Pablo Carreño Busta. Anderson will play his first Grand Slam final at 31.CreditChang W. Lee/The New York Times

“I was playing so-so at the beginning of the tournament,” Nadal, 31, said, “and I have been playing better and better every day.”

That does not bode particularly well for the oft-injured Anderson. He possesses a powerful serve that could theoretically trouble Nadal in their fifth head-to-head encounter on Sunday. But Nadal won their four previous meetings and has the experience of winning 15 major titles, second only to Federer’s 19. He also earned his 15th consecutive Grand Slam semifinal victory, a streak that began after del Potro beat at the U.S. Open in 2009.

He has not been perfect during his run in New York, but he is getting stronger, combining his relentless defense with an increasingly sharp serve and the shrewd tactics of a veteran clay-court specialist.

“He played so smart from the second set till the end of the match,” del Potro said.

After a first set in which Nadal seemed reluctant to hit the ball to del Potro’s high-powered forehand, Nadal summoned his strategic acumen to change the course of the match.

He had been peppering del Potro’s weaker backhand cross court with his lefty forehand, but del Potro was equal to the challenge, at least in the first set. He camped out on the ad side of the court and waited for the right moment to hit penetrating backhands down the line.

Nadal was frustrated, but between sets, he switched to Plan B. No matter how frightening del Potro’s forehand might be, Nadal would have to hit to that side, if only to erase his predictability and force del Potro to move. No longer able to stand in the corner and set his feet, del Potro struggled. He had also just come off two difficult matches in the previous rounds and an illness, and Nadal made him pay for all if it.

As del Potro wore down, he had increasing difficulty with his forehand and his backhand, the shot most affected by his wrist injuries.

“Most important thing is, after the first set, in my opinion, he didn’t hit balls in a row from good positions,” Nadal said. “That makes the difference.”

During one stretch, Nadal won nine consecutive games and 10 out of 11 in the second and third sets. While del Potro staggered to and fro, Nadal only gained strength, urging himself on with repeated fist pumps. With one more win, he will match Federer’s two improbable major titles in 2017.

“It is probably the most important match for me that remains this year,” Nadal said.

While Nadal consolidated his hold on the No. 1 ranking, Anderson, at No. 32, is the lowest-ranked player to reach a U.S. Open final since the inception of the rankings system in 1973.

Anderson and Carreño Busta do not have the name recognition of Nadal or even del Potro, so the audience in Ashe Stadium was relatively subdued during their match.

But it did not matter to the emotional Anderson, who also became the lowest-ranked finalist at a major since 38th-ranked Jo-Wilfried Tsonga at the 2008 Australian Open.

Anderson was already the first South African to reach a U.S. Open men’s semifinal since Johan Kriek in 1980. Cliff Drysdale was the last player from South Africa to reach the final at the U.S. championships, in 1965. He lost to Manuel Santana of Spain.

After he beat Carreño Busta, Anderson raised his arms, took off his white cap and then climbed into his team box to celebrate in a fashion usually reserved for the championship round.

“I will be playing for a Grand Slam trophy,” Anderson said. “That’s an amazing feeling. I have to get ready. Still obviously a very difficult match ahead of me, but I think right now I’m just trying to unwind a little bit and just enjoy sort of today’s match.”

Correction:

An article on Saturday about the men’s semifinals at the United States Open misidentified the last man representing South Africa to reach a Grand Slam final before Kevin Anderson at this year’s U.S. Open. The last to do so was Kevin Curren, at the 1984 Australian Open — not Johan Kriek, at the 1981 Australian Open.

Correction:

An earlier version of a homepage summary with this article misspelled the surname of the tennis player who lost to Kevin Anderson in a United States Open semifinal. He is Pablo Carreño Busta, not Bust.

A version of this article appears in print on , on Page D1 of the New York edition with the headline: In the Men’s Final, It Will Be the No. 1 Seed vs. No. 28. Order Reprints | Today’s Paper | Subscribe